This invention relates to tri-titanium aluminide alloys.
Titanium alloys have found wide use in gas turbines in recent years because of their combination of high strength and low density, but generally, their use has been limited to below 600.degree. C. by inadequate strength and oxidation properties. At higher temperatures, relatively dense iron, nickel, and cobalt base super-alloys have been used. However, lightweight alloys are still most desirable, as they inherently reduce stresses when used in rotating components.
While major work was performed in the 1950's and 1960's on lightweight titanium alloys for higher temperature use, none have proved suitable for engineering application. To be useful at higher temperature, titanium alloys need the proper combination of properties. In this combination are properties such as high ductility, tensile strength, fracture toughness, elastic modulus, resistance to creep, fatigue, oxidation, and low density. Unless the material has the proper combination, it will fail, and thereby be use-limited. Furthermore, the alloys must be metallurgically stable in use and be amenable to fabrication, as by casting and forging. Basically, useful high temperature titanium alloys must at least outperform those metals they are to replace in some respects and equal them in all other respects. This criterion imposes many restraints and alloy improvements of the prior art once thought to be useful are, on closer examination, found not to be so. Typical nickel base alloys which might be replaced by a titanium alloy are INCO 718 or INCO 713.
Heretofore, a favored combination of elements for higher temperature strength has been titanium with aluminum, in particular alloys derived from the intermetallic compounds or ordered alloys Ti.sub.3 Al (alpha 2) and TiAl (gamma). Laboratory work in the 1950's indicated these titanium aluminide alloys had the potential for high temperature use to about 1000.degree. C. But subsequent engineering experience with such alloys was that, while they had the requisite high temperature strength, they had little or no ductility at room and moderate temperatures, i.e., from 20.degree. to 550.degree. C. Materials which are too brittle cannot be readily fabricated, nor can they withstand infrequent but inevitable minor service damage without cracking and subsequent failure. They are not useful engineering materials to replace other base alloys.
There are two basic ordered titanium aluminum compounds of interest--Ti.sub.3 Al and TiAl which could serve as a base for new high temperature alloys. Those well skilled recognize that there is a substantial difference between the two ordered phases. Alloying and transformational behavior of Ti.sub.3 Al resemble those of titanium as the hexagonal crystal structures are very similar. However, the compound TiAl has a tetragonal arrangement of atoms and thus rather different alloying characteristics. Such a distinction is often not recognized in the earlier literature. Therefore, the discussion hereafter is largely restricted to that pertinent to the invention, which is within the Ti.sub.3 Al alpha-two phase realm, i.e., about 75Ti-25Al atomically and about 86Ti-14Al by weight.
With respect to the early titanium alloy work during the 1950's, several U.S. and foreign patents were issued. Among them were Jaffee U.S. Pat. No. 2,880,087, which disclosed alloys with 8-34 weight percent aluminum with additions of 0.5 to 5% beta stabilizing elements (Mo, V, Nb, Ta, Mn, Cr, Fe, W, Co, Ni, Cu, Si, and Be). The effects of the various elements were distinguished to some extent. For example, vanadium from 0.5-50% was said to be useful for imparting room temperature tensile ductility, up to 2% elongation, in an alloy having 8-10% aluminum. But with the higher aluminum content alloys, those closest to the gamma TiAl alloy, ductility was essentially non-existent for any addition.
During the 1960's and 1970's considerable work was done by and for the U.S. Air Force covering the Ti-Al-Nb system. In U.S. Pat. No. 4,292,077. "Titanium Alloys of the Tl.sub.3 Al Type". Blackburn and Smith identify 24-27 atomic percent aluminum and 11-16 atomic percent niobium as the preferred composition range. High aluminum increases strength but hurts ductility. High niobium increases ductility but hurts high temperature strength. Vanadium is identified as being able to be substituted for niobium up to about 4 atomic percent.
In U.S. Pat. No. 4,788,035, "Tri-Titanium Aluminide Base Alloys of Improved Strength and Ductility", Gigliotti and Marquardt disclose a Tl.sub.3 Al base composition having increased tensile strength, ductility and rupture life due to the addition of Ta, Nb and V.
Nb alone has been used as a principal beta phase promoter in Ti.sub.3 Al. As noted previously, V can be substituted for Nb up to about 4 atomic percent. We found that rapidly solidified Ti.sub.3 Al alloy containing 12 atomic percent Nb was somewhat ductile at room temperature due to its alpha two plus beta two structure. However, the alloy became brittle after exposure above 750.degree. C. due to conversion of the beta two to alpha two.
Accordingly, it is an object of the present invention to provide a Ti.sub.3 Al alloy having room temperature ductility and high temperature strength.
Other objects and advantages of the present invention will become more apparent from the following description of the invention.